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Discussion Starter #1
I am experimenting a little with my LLT sonotubes, and tried laying them on their sides. So for the first time I've gotten a good look at the drivers while they are working. Did measurement sweeps today, and some pink noise. I was curious as to how much movement I got when pushing the tubes a little, so I decided to have a look while the sweeps ran. I saw virtually no movement at 75dB. So I decided to take them to reference. I've seen many movies at reference and knew the subs could do it fine, I just wanted to get a look at that 2" excursion.

Well...

Turns out, even at reference doing sine tones or pink noise, there's not alot of excursion. At 13Hz, [email protected] and -3dB sine wave I had an estimated 1cm of excursion. The house was rocking and walls were bending, and I measured around 110dB @LP. Are the LLT tubes really that efficient? My FR is reasonably flat from around 13 up to 80 with a dip at 20, and even doing pink noise there was little movement.

So, no movies can ever go over 0dB, right? So I'm really THAT safe when it comes to excursion and power? Am I limiting my output somehow? I have used Audyssey to calibrate, and like I said, measure flat down to 13Hz. I've compared Audyssey to the raw resonse, and it really don't bring the peaks down as much as bring the overall level up to match the peaks. I guess Audyssey somehow know my subs are capable of that. Still, I was amazed at how little work they had to do to achieve reference levels. :huh:
 

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Elite Shackster
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As the frequency produced rises, cone excursion required at a given spl reduces. Its the low level stuff that requires high excursion. A ported sub give very clean output at around the tuning frequency (usually very low down the range) because the driver dosent have to move much, and higher excursion is what introduces distortion (among other factors). A well designed ported sub should never really have to move all that much, certainly not like a sealed sub does.

If you look at a simulation of your subs, you can see projected cone excursion at given frequencies under maximum power. Most of the time, even at reference level, your subs wont need that much power, its only required in spikes, and pink noise wont cause the limits to be hit.

Stress testing can be dangerous, so be aware then doing anything like that. Also, Ive seen people become obsessed about the drivers not moving anything like they are capable, and wanting to see them do that, but a good design is intended to reduce excursion, and just because the drivers can move loads, doesnt mean they have to, the all round design is what will dictate how much they generally move, and your drivers would be working like the clappers in a sealed sub in comparison.

If you want to test the drivers (and you do so at your own risk), then look at the excursion chart in the simulation (I assume you ran one to design your subs), and there will be a particular frequency where excursion is highest. Then use a tone at that exact frequency and increase the volume till you see the drivers moving, and this will get them moving the most. You wont really prove anything by doing this so its kinda pointless, but if you want to see the drivers moving, thats your best chance. If your design only shows the drivers at half xmax at full power at the frequency of highest excursion though, thats the most movement your ever going to see without more power from somewhere, and you could then cook the voice coil, so I dont recommend that either if your unsure about what your doing.
 

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Discussion Starter #6
With an "accurate" SPL meter, i.e. the RadioShack meter that 'everybody' uses. I know it's not super accurate, but it's ballpark. And with the Audyssey calibration you are supposedly at reference with [email protected] I measure 73dB with the processor pink noise for each channel and [email protected]

I wasn't really looking to see the drivers move just for the sake of it, I was doing it for peace of mind, really. If I know the stress test is barely tickling my subwoofers then I shall fear no LFE track henceforth. :thumb:
 

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The reason I asked is that many assume the display number is always related to the actual acoustic level, and often times it is not. Can't comment re Audessy as I have not used it, though I will when I get the new AVR.

The other thing you've got from this is some experience with the relationship of numbers on a screen with what something is actually doing in practice. Earlier this year I got this put back in perspective for myself again: I have a friend with a 2ch system using 18" drivers for midbass (sealed), measure flat at LP to 35Hz in a large room with no gain, listening very, very loud to bass dense music (real music not SFX show off stuff) I observed the 18's moving no more than 2mm. Drop an octave and you need 4x displacement for the same SPL, so 8mm and not too far off your own observed excursion. < I know that is all very rough for comparison, but as it is not several times different, close enough for discussion.

If it is loud enough for you and you are only using a small fraction of your system's displacement, then great as you are not taxing the drivers and have lower distortion as a result.
 
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